Biodiesel Fundamentals For High School Chemistry Classes

Biodiesel Fundamentals for High SchoolChemistry ClassesLaboratory 5: Transesterification of Vegetable Oil andAlcohol to Produce Ethyl Esters (Biodiesel)Topics CoveredxTransesterification of vegetable oils or animal fats as a reversible chemical reactionxThe role of catalyst in transesterification of vegetable oils or animal fatsxThe use of ethanol (rather than the more common methanol) in biodiesel productionxThe importance of temperature and agitation/mixing for the chemical reactionxSide reactions competing with transesterificationxSeparation of co-product glycerolEquipment Needed (per pair or group)xxxxxxx250 mL flask or beaker, and small watch glass or stopper125 mL Erlenmeyer flask and stopperSeparatory funnel, 250 mLStirring hot plate and 2 magnetic stir barsThermometerAluminum foilWeighing scale (readable to 0.01 g)Reagents Needed (per pair or group)xxxEthanol (anhydrous, 100%)1 gram potassium hydroxide100 grams vegetable oil, tallow, or lard (food grade)Background Information -- TransesterificationAccording to the current American Standards for Testing Materials (ASTM), biodiesel isdefined as the mono alkyl esters of long chain fatty acids derived from vegetable oils or animal fats,for use in compression-ignition (diesel) engines.One process for making biodiesel is called transesterification. This is a chemical reaction inwhich 1 mole of triglycerides (vegetable oil or animal fats) reacts with 3 moles of alcohol to produce 3moles of alkyl esters (biodiesel) and 1 mole of glycerol, a co-product. This is a reversible, equilibriumreaction and requires excess alcohol to push it toward the products side. Usually, 6 moles of alcoholare used per 1 mole of triglycerides. Le Chatelier's principle explains this behavior. This principle,named after Henry Le Chatelier, can be summarized as follows:37Biodiesel Education Program Biological and Agricultural Engineering University of Idaho

Biodiesel Fundamentals for High SchoolChemistry ClassesIf a chemical system at equilibrium experiences a changein concentration, temperature, volume, or partial pressure, then the equilibrium shiftsto counteract the imposed change and a new equilibrium is established.Typically, a strong base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) isused as the catalyst, which normally would not be consumed during this reaction and would end upin the glycerol layer. This is not always the case, as we will see later.OOH2-C – O – C – R1OH –C – O – C – R2O 3 C2H5OHH2-C – O – C – R3C2H5 – O – C – R1OH2-C – OHC2H5 – O – C – R2 OH –C – OHC2H5 – O – C – R31 Canola Oil3 Ethanol3 Ethyl Ester sH2-C – OH1 GlycerolFigure 1 - The Transesterification ReactionA triglyceride is a molecule made up of three fattyacids attached to a glycerol backbone. Figure 1 shows adepiction of the reaction where the R’s represent the fattyacids that make up a triglyceride. The three fatty acids oneach molecule could be the same or they could be different.Each oil or fat has a unique fatty acid profile. Canola, forexample, is high in oleic acid which is an 18 carbon chainwith one double bond. It also contains linoleic acid as well asa number of other less prominent fatty acids.The fatty acid makeup of the oil or fat is responsiblefor some of the properties found in the resulting biodiesel.The more saturated (no double bonds) the oil or fat is, themore stable the biodiesel (the less likely it is to go rancid).However, the cold flow properties tend to be poorer as thesaturation increases. In other words, saturated fats tend tosolidify at higher temperatures than do unsaturated oils.These trends are reversed as the level of unsaturation(number of double bonds) increases. Notice, in the pictures to the left, that unsaturated fatty acids arebent, and therefore do not readily stack up to form a crystalline structure. These crystals are often38Biodiesel Education Program Biological and Agricultural Engineering University of Idaho

Biodiesel Fundamentals for High SchoolChemistry Classesassociated with filter plugging during cold weather. On the other hand, the saturated fatty acids arestraight and will stick together to form crystals at higher temperatures, causing biodiesel made fromsaturated fats to gel (and clog up the engine) even at temperatures significantly above freezing.You can see in Figure 1 that it takes three molecules of ethanol for each molecule oftriglyceride to satisfy a complete reaction. This would be a 3:1 molar ratio of alcohol to oil. Typically inpractice, a 6:1 molar ratio is used to drive the reaction to the product side. What happens to the excessalcohol that was not used in the reaction? It is unused at the end of the process and separates partlywith the fuel and partly with the glycerol. It can be recovered and reused through a process ofdistillation.The transesterification reaction will proceed quickly at first and eventually slow down as itreaches equilibrium. At this point, there will be some unreactedglycerides left in the biodiesel. It is possible to improve thequality by removing the glycerin that has settled out and to runanother reaction, thus pushing the equilibrium point further tothe products side. For this lab, one reaction should be enough toget a good separation of the glycerin from the biodiesel.BiodieselBBiiododidieieseiesesell cacann bebe mademadadede frffromromTo calculate the mass of alcohol needed to achieve a ockckss.molar ratio, you will need the molecular weights of the triglyceridessand the alcohol being used. In this example, canola oil and ethanol are the reactants.It can be very difficult to calculate the molecular weight of a fat or oil, since they are made upof many different large molecules (trigylcerides), each with different fatty acids attached. For thisexercise we will use a value of 877 g/mol for canola.Ethanol is not completely soluble in fats and oils, so vigorous mixing is required. As thereaction proceeds the product (alcohol esters) acts as a co-solvent, making the two reactants miscibleimproving contact and interaction.For most chemical reactions, temperature plays a major role. There is a rule of thumb thatstates “for every 10 C rise in temperature the speed of the reaction doubles.” To increase the reactionrate, it’s relatively easy to increase the temperature. For an unpressurized reactor the temperatureshould not exceed the boiling point of the alcohol (78 C for ethanol) for the obvious reason that theethanol will boil away and the reaction will stop and in some cases reverse. A pressurized reactor,although requiring specialized construction, would allow for a higher temperature reaction and thus afaster reaction rate.Sometimes an unfortunate side reaction called saponification occurs. This is the process ofmaking soap. Whenever triglycerides (oils and fats), a strong base (sodium or potassium hydroxide),and water come together, soap is formed. Oils and fats used for cooking tend to oxidize due to hightemperatures and contact with water. This can cause some of the fatty acids to break off the glycerolbackbone and float free. These are referred to as free fatty acids (FFA) as shown in Figure 2. They willquickly react with the catalyst to form soaps, destroying some of the catalyst in the process. This39Biodiesel Education Program Biological and Agricultural Engineering University of Idaho

Biodiesel Fundamentals for High SchoolChemistry Classesreaction proceeds much faster than transesterification. In this case, extra catalyst will be needed tocarry out the transesterification reaction. In the case of oils with very high FFA, an acid or enzymecatalyst has to be used to cause the esterification of the FFAs to biodiesel.OOR1– C – O – HFFA K-OHBaseR1– C – O – KSoap H-OHWaterFigure 2 - The Saponification ReactionWhy is this unfortunate? Soap is a long molecule that has a polar head and a non-polar tail.While polar substances are not usually soluble in non-polar solvents, and vice-versa (like dissolveslike), soap has properties of both, making it a perfect substance to clean non-polar oil and grease offyour hands using water. However, soap can complicate the separation of the non-polar esters from thepolar glycerol, water and alcohol.Normally, glycerol, water and alcohol are not very soluble in biodiesel. They tend to hang onto each other and separate quickly by gravity from the biodiesel due to their higher combined density.However, the more soap that is present, the more these compounds become soluble in each other. Thismakes it harder for them to separate, which causes a loss of yield (less biodiesel produced). If a smallamount of water is added at the end of the reaction it can shift the phase equilibrium in favor of amore complete separation.Pre-Lab Exercises1.2.3.Log on to the triglyceride molecular weight lecularCalculator.aspCheck the molecular weights of the fats and oils in the pull-down menu.Compare the molecular weight of rapeseed oil to that of palm kernel oil. What differences doyou see in the fatty acid make-up of these two? Why do think there is a big difference inmolecular weights of these two triglycerides?Safety Note: Because of the potentially dangerous chemicals used in this lab, everyone must wearsafety glasses or goggles while working in the laboratory. Lab coats or aprons should be worn whenpouring or mixing chemicals. If a fire occurs, leave the building immediately. Do not try to fight it!40Biodiesel Education Program Biological and Agricultural Engineering University of Idaho

Biodiesel Fundamentals for High SchoolChemistry ClassesLaboratory ProcedureThe following procedure will be used to make a small batch of biodiesel1.2.Calculate the molecular weight of ethanol.Since we are using 100 g of oil, we can use the following relationship to calculate the ethanolneeded for a 6:1 ratio of methanol moles to triglyceride moles:100 g of oil877 g/mol X g of ethanol6 *( 46 g/mol)3.Following safe lab procedures, weigh out 1 g of potassium hydroxide into a 125 mLErlenmeyer flask. Into this same flask add the calculated amount ethanol (from step b).4.Add a magnetic stir bar, cover with aluminum foil or a watch glass, and place on a stirring hotplate. Mix until the KOH is completely dissolved. Set this aside and go to the next step.5.Weigh an empty 250 mL Erlenmeyer flask and record the weight intable 1. Into this same flask weigh out about 100 g of the oil or fat tobe converted to biodiesel. Again record the exact weight in table 1.Add a stir bar, put on a hot plate, and heat to around 60 C.6.Next, add the alcohol/catalyst mixture to the oil in the flask andcover with a watch glass or aluminum foil to keep the alcoholvapors from escaping. Stir this mixture at a moderate level for 60minutes while holding the temperature at 60 C.7.Transfer the mixture to a separatory funnel and allow it to settle for20 minutes.8.Remove the glycerin (lower) layer; the upper phase is the biodiesel.9.Note: If there is no phase separation after 5 to 10 minutes, it can be forced by the addition ofwater. Return the mixture to the 250 mL Erlenmeyer flask, and while stirring, add 15 mL ofwater and continue to stir for about 10 seconds. Then, return the mixture to the separatoryfunnel. This action will concentrate the polar molecules of water, ethanol and glycerol andallow them to settle along with a good portion of the soap.10. Weigh a clean 250 mL beaker or 250 mL Erlenmeyer flask and record the weight in table 1.Next weigh the clean biodiesel and record that weight to be used for the yield calculation.41Biodiesel Education Program Biological and Agricultural Engineering University of Idaho